Omar Mateen, 29, was 'on the radar' of U.S. officials for some time

The gunman who slaughtered at least 50 people inside a gay club in Orlando called 911 and swore allegiance to the leader of Islamic State shortly before the massacre, according to law enforcement officials.

Shooter Omar Mateen, 29, from Port St. Lucie in Florida, opened fire at Pulse night club in the early hours of this morning.

At least 50 people were killed and 53 others were injured in the shooting in the deadliest mass shooting in US history. A surgeon at Orlando Regional Medical Center said the death toll was likely to climb.

But shortly before the attack, Mateen, who was born in New York to Afghan parents, called emergency services and pledged allegiance to Abu Bakr al-Baghdadi, law enforcement officials told NBC News.

California congressman Adam Schiff.Schiff, who serves as top Democrat on the House Select Committee on Intelligence, told CNN: ‘What I’ve heard from the Department of Homeland Security this morning is that, according to local police, he made a pledge of allegiance to ISIL (ISIS).’

He said the shooting was ‘highly indicative of an ISIL-inspired attack’.

However his father, Mir Seddique, told NBC News his son became angry when he saw two men kissing in Miami several months ago.

‘This has nothing to do with religion,’ he said. ‘We are saying we are apologizing for the whole incident. We weren’t aware of any action he is taking. We are in shock like the whole country.’

An FBI spokesman said the mass shooting is being investigated as an act of terrorism, adding that they are looking into whether this was an act of domestic or international terror, and if the shooter was a lone wolf.

US citizen Mateen, who was shot dead by officers, entered the nightclub wielding an AR-15 assault rifle and a handgun.

Writing on Facebook, Pulse, which described itself as the most prominent gay club in Orlando, urged party-goers to ‘get out and keep running’ as bullets started flying at around 2am local time.

Orlando Police Chief John Mina said the suspect exchanged gunfire with an officer working at the club around 2am, then went back inside and took hostages. There were about 320 people inside the club at the time of the shootings and about 100 people were taken hostage.

At around 5am authorities sent in a SWAT team to rescue the hostages. Nine hero officers used a ‘controlled explosion’ to distract the shooter before fatally shooting him and were able to rescue about 30 hostages who were hiding in the bathroom of the club.

During the gunfire, an officer was shot, but he was saved by his helmet. It was thought that at least one hostage had been locked in a bathroom with gunshot wounds.